Multielectron Redox Chemistry of Transition Metal Complexes Supported by a Non-Innocent N3P2 Ligand: Synthesis, Characterization, and Catalytic Properties

摘要

A new redox-active, diarylamido-based ligand (L-N3P2) capable of (5)-N,N,N,P,P chelation has been used to prepare a series of complexes with the general formula [M-II(L-N3P2)]X, where M = Fe (1; X = OTf), Co (2; X = ClO4), or Ni (3; X = ClO4). The diarylamido core of monoanionic L-N3P2 is derived from bis(2-amino-4-methylphenyl)amine, which undergoes condensation with two equivalents of 2-(diphenylphosphanyl)benzaldehyde to provide chelating arms with both arylphosphine and imine donors. X-ray structural, magnetic, and spectroscopic studies indicate that the N3P2 coordination environment generally promotes low-spin configurations. Three quasi-reversible redox couples between +1.0 and -1.5 V (vs. Fc(+)/Fc) were observed in voltammetric studies of each complex, corresponding to M-II/M-III oxidation, L-N3P2-based oxidation, and M-II/M-I reduction (in order of highest to lowest potential). Spectroscopic and computational analyses of 3(ox) - generated via chemical one-electron oxidation of 3 - revealed that a stable diarylaminyl radical (L-N3P2) is formed upon oxidation. The ability of the Co-II complex (2) to function as an electrocatalyst for H-2 generation was evaluated in the presence of weak acids. Moderate activity was observed using 4-tert-butylphenol as the proton source at potentials below -2.0 V. The insights gained here will assist in the future design of pentadentate mixed N/P-based chelates for catalytic processes.